Is This Going to Be a Stand-Up Fight, Sir, or Another Sloth Hunt?

Harlan's ground sloth at the Royal Tyrrell Museum in Alberta. Credit: Brian Switek

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White Sands National Monument is a strange place. Hemmed in by military installations, it’s the only national park I know of that’s had to cope with “errant missiles.” The park also tangles with the ravenous appetites of African oryx, introduced as game animals in the 1960s and finding New Mexico perfect to their tastes. But perhaps the oddest aspect of this remote park is what remains of its distant past. During the Pleistocene, over 12,000 years ago, mammoths, saber-toothed cats, and other megafauna left their footprints around an enormous, ancient lake. Those traces remain, weathering out of the arid desert flats, and, among this fantastic aggregation of tracks, paleontologists have what was either an Ice Age hunt or the Pleistocene equivalent of cow tipping.

The tracks, described by park naturalist David Bustos and colleagues, were found near the margin of one of the park’s alkali flats. About 30,000 years ago, this now-desolate place was part of Paleo-Lake Otero. That water source not only attracted life, but provided just the right conditions for their footfalls to be preserved. In this case, a strange interaction between prehistoric people and a giant ground sloth.

Seeing the tracks together, I had to chuckle a little. Over a century ago, giant ground sloth tracks found in Carson City, Nevada made news because their crescent shapes looked like the footprints of a human giant. If the White Sands National Monument tracks at the center of the new paper had been found earlier, perhaps Mark Twain would have been lampooning these fossils rather than those found at the Carson City prison. As it stands, though, the scientific analysis has something even more spectacular to share. The tracks of the giant ground sloth and the Pleistocene people show how these Ice Age inhabitants reacted to each other.

Trace fossils are recordings of prehistoric behavior. This is true whether paleontologists are looking at a single footfall or an entire trackway. But finding interactions between organisms - and particularly different species - is rare, particularly in this case where humans have often been implicated in the demise of charismatic mammals like the shuffling ground sloths. (Giant sloth bones found in Uruguay supposedly show signs of cut marksmade by humans, for example, but the evidence is disputed.) The tracks present a few fleeting moments of Pleistocene life.

So what happened? Sussing out the story isn’t as simple as just following one foot in front of the other. Determining the window in which tracks were formed is critical to determining interactions. In this case, there are over a hundred sloth and human tracks representing several individuals of each group. This wasn’t a gang of humans running up to a single sloth. It seems that the prehistoric people either harassed a group of sloths or bothered multiple sloths in close succession. Whenever the human tracks come in close proximity to the sloth tracks, Bustos and colleagues write, the sloth traces seem to show evasive and defensive behavior. There are even tracks that look like places where the sloths stood up on their hind legs into a defensive posture similar to what giant anteaters do today.

The question is what these Pleistocene people were doing. Bustos and coauthors are right that sloths were not to be trifled with. The megamammals had thick skin, toughened further by pebbles of bone, and each of these mammals had burly arms tipped in sharp claws that “gave them a lethal reach and clear advantage in close-quarter encounters.” Hunting giant sloths would have been a fool’s errand, if that’s even what these people were doing.

There’s no giant sloth body at the end of the trackways. Nor are there tracks and traces showing a sloth being felled. Tools and weapons prehistoric people would have used are also absent from the site. We know the people got close enough to bother the sloths, but there’s no direct evidence for hunting. Only the assumption that prehistoric people might have done so. But there’s another set of tracks that speaks to an alternative, even if it’s almost impossible to confirm.

Human tracks don’t just surround the sloth tracks. Some of the human tracks fall within the giant sloth footprints. Ease of passage - like a person following a game trail - doesn’t make sense. This was a lake margin, not a closed-in habitat. And, as Bustos and colleagues write, it would have actually been a little uncomfortable for the person to do this. Their stride length was shorter than that of the sloth, so they would have needed to take an unnatural gait to match up with the prints - just like I do with some dinosaur trackways out in the desert. That kind of ichnological fun might be the key to a scenario that contrasts with the violent confrontations depicted in news reports on this paper.

“It is possible that the behavior was playful,” Bustos and colleagues write, the Pleistocene person following in the footsteps of the sloths just because they could. Maybe the entire encounter was a matter of Pleistocene fun or even a dare, harassing the sloths more for entertainment than food. Admittedly, this is an untestable hypothesis. The tracks don’t testify to this possibility. But neither should hunting be taken as the default in the absence of evidence to support it. Do the White Sands tracks represent an unsuccessful hunt? Maybe. But it’s just as likely that they confirm the long history of people being jerks to animals.

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